Conventionally, an efficiency in collecting photocharges is controlled by using a photogate of a MOS structure and the distance from an object can be measured by using the phase difference between emitted light and reflected light. However, in the MOS-type photogate structure using poly-silicon (poly-Si), a light absorption factor is generated in the course of the measurement and thus loss of light efficiency may be generated. When the thickness of poly-Si is made thin in order to restrict the loss, the resistance of poly-silicon increases so that the voltage applied to the photogate may not be sufficiently transferred.
Also, in the MOS type photogate structure, noise due to dark current may be generated because photocharges concentrate at the boundary surface between Si and SiO2. Furthermore, the MOS type photogate structure can require a high voltage of 3.3V or higher for stable operation thereof.